I've been cleaning (99% of jobs wfp) since April. My experience is that it can take a few cleans to get the windows spotless on some properties. I don't think it's a con. We wfp cleaners can usually deliver a very acceptable clean first time round on just about all jobs however I have experienced filthy vents that drip water after you've cleaned (even after you have gone away to another window and then returned to rinse again). It's great to see algae running away from the glass and frames. I always warn that grey stains won't come off without a proprietary cleaner - people tell me not to worry. I price some first cleans to allow me to take the time necessary to give a great result.
From the ground and looking high up it's hard to actually see fly poo and bee poo / snail trails, so some of these might not be cleaned thoroughly first time - they don't come off with a porcupine applicator or the scouring end of an Unger monsoon applicator either and i'd be willing to wager that trad guys doing 25 a day don't scrape every such window... Hydrophobic glass always makes wfp cleaning harder. I'd ban the damned stuff.
As you clean a second or third time vents come clean (I wash them every visit to gradually get them clean); first time though (and maybe with tfr) fly poo comes off with a bit of elbow grease; ditto snail trails. It's just high up that they might be a problem.
This is not a dig at trad guys - biggup the ladder posse - but they generally don't do the frames. I spend time getting as much spider webs out of the corner (and I mean where the bricks abutt the frame, not where the glass meets the frame, i.e. the reveal). Wfp cleaners could have a go at them saying they are glass cleaners, not window cleaners, but this is the quick route to disaster in my book - we shouldn't be fighting. If custies want a trad guy to do the frames - he probably will, but will rightly charge them for it, so the clean will be double the price at least; algae covered and they are gonna need a mortgage for me to do a house trad! It just happens that wfp cleaners do the frames as well.
In my book, in competent hands, a good job for an acceptable price is delivered by both methods, from the first clean.